1. Field of the Invention
The present invention relates to signal processing carried out in an image capture apparatus such as a digital camera. Particularly, the present invention relates to techniques of pixel interpolation.
2. Description of the Background Art
In an image sensor such as a CCD image sensor and a CMOS image sensor which are used in an image capture apparatus such as a digital camera, light received via a color filter is converted into an electrical signal through photoelectric conversion, to be output as a pixel signal. As an example of such a color filter, an RGB-type color filter and a YMCK-type color filter can be cited. Then, a single image sensor outputs a pixel signal of a single color for a single pixel. For example, in a case where an RGB-type color filter is employed, a pixel signal of any color component out of red (R), green (G), and blue (B) color components, is output for one pixel.
Hence, for a pixel signal output from a single color image sensor, there is a need to obtain pixel signals of the other color components by an interpolation process. An interpolation process uses various algorithms. For example, according to one possible method, a degree of correlation in a horizontal direction and a degree of correlation in a vertical direction are calculated, and an interpolation process is carried out using pixels arranged in the horizontal direction or the vertical direction, in whichever there is stronger correlation. Also, according to another possible method, weighting is conducted in accordance with a distance between an observed pixel and its peripheral pixel, and then, an interpolation process is carried out.
According to techniques disclosed in Japanese Patent Application Publication No. 2006-186965, in an image captured by an image sensor with Bayer array (RGB-color filters), a gray area and a color area are distinguished from each other, and a kind of a pixel interpolation process which is suitable for the characteristics of each of the areas in the image is applied. Particularly, according to the techniques of Japanese Patent Application Publication No. 2006-186965, it is designed so as to suppress occurrence of a false color in an area of a border between a gray area and a color area. More specifically, according to the techniques of Japanese Patent Application Publication No. 2006-186965, for the area of the border between a gray area and a color area, a process of determining a correlation direction is carried out using a method of determining which is suitable for a gray image. On the other hand, a pixel interpolation process is carried out using a method of pixel interpolation which is suitable for a color image. This can suppress occurrence of a false color which is likely to occur by a pixel interpolation process, in the techniques of Japanese Patent Application Publication No. 2006-186965.
As described above, the techniques of Japanese Patent Application Publication No. 2006-186965 is predicated on the use of an image sensor with Bayer array (RGB-color filters). Thus, there is no description about specific techniques for carrying out a suitable pixel interpolation process on a pixel signal output from an image sensor with three-color filters in arbitrary colors which are different from R, G, and B.
Then, in view of the foregoing problems, it is an object of the present invention to implement a pixel interpolation apparatus, an image capture apparatus, a program, and an integrated circuit which can appropriately carry out a pixel interpolation process on an image signal captured by a single image sensor with three-color filters, in whatever pattern the three-color filters may be arranged.